Method of heating and producing oil wells



April J. O'BRIEN 3,087,545

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LEO .1. O'BRIEN ATTORNEY United States Patent 3,087,545 METHOD OFHEATING AND PRODUCING OIL WELLS Leo J. OBrien, Crystal Lake, 11].,assignor to The Pure Oil Company, Chicago, 111., a corporation of OhioFiled Aug. 9, 1961, Ser. No. 130,406 6 Claims. (Cl. 16639) Thisinvention relates to a novel method for increasing the production ofpetroleum from wells subject to plugging by waxy, petroliferous depositswhich form in the tubing string through which petroleum is produced. Ina particular aspect, this invention relates to a method of assisting theproduction of petroleum from wells by gaslift.

In certain oil reservoirs, elude oils are produced which contain largeamounts of wax. Under virgin reservoir conditions, the wax is containedin solution in the res ervoir fluid. When the oil is produced through awell under conditions of reduced pressure, wax deposits form in thetubing string by which the petroleum is conveyed from the producinghorizon to the surface of the earth. The flow capacity of such wells isthereby reduced to a very low value. While periodic cleaning of the welltubing temporarily alleviates the situation, deposits continue to formin the tubing string and plugging again occurs. The problem isespecially severe in low-pressure reservoirs wherein there may beinsuflicient pressure to raise the petroleum to the surface of theearth, even under ideal conditions of flow.

It has been proposed to heat wells subject to plugging by paraffindeposits by means of electric heaters, or by pumping a combustiblemixture of gas and air down the well annulus to a burner, and returningthe combustion products through the well annulus to the surface of theearth. It has further been proposed to increase the production fromlow-pressure reservoirs by pumping a pressurized gas, such as methane orair, down to the bottom of the well, and there introducing the gas, inproper amounts, into the oil column standing in the petroleumproducingtubing string, so that the gas exerts a lifting force on the petroleum.This technique is commonly referred to as gas-lift.

In some instances, where waxy oils are produced from apetroleum-containing stratum, it is found that a subadjacent orsupper-adjacent stratum exists which is permeable, but separated fromthe petroleum-producing straturn, by an impermeable layer of rock. Insome instances, the permeable super-adjacent or sub-adjacent formationcontains quantities of non-commercial combustible materials.

It is an object of this invention to provide a method for employinglow-grade combustible materials produced from a formation adjacent to anoil-producing stratum as a source of fuel from which heat may beobtained to prevent the deposit of petrolifer-ous materials in thepetroleum production tubing string. Another object of this invention isto provide a method for heating the petroleum in an oil tubing-string bydirect heat exchange with combustion products. Still another object ofthis invention is to provide a method by which combustion products maybe employed to achieve gas-lift. Yet another object of this invention isto provide a method by which the pumping costs of heating a well bore bymeans of a fuel burner and of providing a gas-lift are reduced.

This invention is best described with reference to the drawings, ofwhich:

FIGURE 1 shows, in section, a petroleum reservoir which is penetrated bya well,

FIGURE 2 is an enlarged sectional view of the element 42 of FIGURE 1,and

3,087,545 Patented Apr. 30, 1963 FIGURE 3 is an enlarged sectional viewof the element 50" of FIGURE 1.

Referring to FIGURE 1, the reservoir comprises a petroleum-producingstratum 10, an impermeable layer 12, and la permeable stratum 14 whichcontains noncommercial, 600-B.t.u.-per-cubic-foot natural gas. To besusceptible to treatment in accordance with this invention, thereservoir must have an impermeable layer with a thickness within therange of about 5 to 50 feet and a permeability which is less than onemillidarcy, and preferably as low as 0.01 millidarcy. The porous,gas-containing stratum preferably has a thickness in the range of about10* to feet, and a permeability which is sufficient to provide a flow ofgaseous fuel from the stratum into a well-bore. The Well casing 16 isperforated at 18 to communicate with the petroleum-producing zone 10.Oil-producing string 20 extends through the well-bore to a pointadjacent to perforations 18. A packer 22 is set at the impermeable layer12 to isolate portions of the well-bore lying above and below thepermeable layer, that is, to separate the gas-producing stratum from theoil-producing stratum. The casing is perforated at 26, adjacent to thegas-containing stratum 14. Packers 28 and 30 are preferably placed aboveand below the perforations 26, although the presence of such packers isnot absolutely essential. A second tubing string 32 is also disposedwithin the well-bore. This tubing string terminates at burner 34, whichis positioned adjacent to the impermeable layer 12. Thus the burner ispositioned in an isolated well-bore zone 36 adjacent to the impermeablelayer. A length of tubing 38 extends from the zone 36 through thewell-bore zone between packers 28 and 30, and into the well-bore zone40' above packer 28.

In the tubing string 32, in the zone between packers 28 and 30, isgas-air manifold 42. Referring to FIGURE 2, this manifold is seen tocomprise a collar 44 and a plurality of gas-induction tubes, such astubes 46 and 48. In the combustion zone 36, where burner 34 is located,a lifting-gas induction manifold 50 is provided in thepetroleum-producing string 20. A second lifting-gas induction manifold50 is placed in the petroleum string 20* in wellbore zone 40 abovepacker 28. Referring to FIGURE 3, the lifting-gas induction manifold isseen to comprise a collar 52 having a central bore 54 of lesser diameterthan the interior of the tubing which comprises the tubing string.Communicating with bore 54 are gas inlets, such as inlets 56 and 58.These inlets are controlled by means of check-valve assemblies 60 and62, respectively. The check valves are of the conventional type, and actto close the conduits 56 and 58 in the event the inlet pressure dropsbelow the predetermined closing pressure of the valves.

In operation, while petroleum is produced from stratum 10, gas iscollected in the zone between packers 28 and 30. Air, or otheroxygen-containing gas, is pumped down the tubing string 32 to gas-airmixing manifold 42, where the gas and air mix to form a combustiblemixture, which flows to burner 34, where combustion occurs. Thecombustion products are released in the Well-bore zone 36, adjacent toimpermeable layer 12, and a portion of these combustion products flowsupward through tubing 38 into the well-bore annulus above packer 28 theremainder of the combustion products entering the petroleumtubing-string 20, at lifting-gas induction manifold 50, to providegas-lift and to bring the combustion products into direct heat exchangewith the petroleum. Additional lifting-gas induction manifolds 50, asmany as desired, may be placed at various intervals in tubing string 20to provide for the introduction of additional quantities of combustionproducts to the tubing string, to compensate for the loss of volume ofthe combustion products which occurs as the combustion products cool byheat exchange with the petroleum in the tubing string. The hotcombustion products serve to warm the petroleum, preventing theformation of waxy deposits and melting deposits already formed.

It will be evident that where the gas produced from stratum 14 is of nocommercial value, and is available in ample quantities such that it neednot be conserved, it will not be necessary to provide packers 28 and 30as shown. The combustion products may merely be allowed to mix withproduced gas and pass up the well-bore in heat-exchange relationshipwith the petroleum tubingstring, portions of the combustion products andnatural gas entering the petroleum string at the lifting-gas inductionmanifolds 50 to provide gas-lift. Further, in some cases it will bedesired to limit the quantity of air pumped down the well-bore to avalue below that suflicient to provide enough combustion products forthe gas-lift. It is desirable to keep the quantity of air as low aspossible, both to reduce pumping costs and to keep temperatures in thewell annulus at an acceptable level, whereby petroliferous deposits aremelted, but deleteriously high temperatures are not encountered. In suchcases, it will be desirable to introduce a portion of the unburnednatural gas produced from the stratum 14 into the petroleumtubing-string 20, to provide additional gas-lift. This can beaccomplished simply by disposing a manifold 50 in the tubing string 20between packers 28 and 30, or by perforating packer 28, so that theproduced unburned gas can flow upward to the manifold 50 disposed abovepacker 28. Where the combustion products are available in quantities inexcess of that required for gas-lift, the excess combustion products maybe vented up the wellbore to the atmosphere. The proper pressure can bemaintained in the well annulus by venting through a check valve, notshown, located at the top of the well casing.

As a specific example of the method of this invention, a wellpenetrating a gas-producing stratum, an impermeable layer, and apetroleum-producing stratum is equipped as shown in FIGURE 1. Gaspressure at the well is 200 p.s.i. One hundred standard cubic feet ofair per hour, at a pressure of 225 p.s.i., are pumped down tubing-string32 and mixed with a stoichiometric quantity of gas. The combustiblemixture is burned in burner 34. One-quarter of the volume of thecombustion products is introduced into petroleum tubing-string 20 atlifting-gas induction manifold 50 in zone 36. The remainder of thecombustion product passes upward through tube 38. At the inductionmanifold in zone 40, a quantity of combustion product amounting toone-eighth of the total is introduced into the petroleum tubing-string.The remainder of the combustion products passes upward through thewell-bore in indirect heat-exchange relationship with the externalsurface of the petroleum tubing-string.

As another specific example of the method of this invention, a wellpenetrating a gas-producing stratum, an impermeable layer, and apetroleum-producing stratum is equipped as shown in FIGURE 1. Gaspressure in the has-bearing formation (6,000 feet subsurface) is 1,000p.s.i. Crude petroleum produced at a rate of 200 barrels per day entersthe tubing string at 6,500 feet subsurface at a temperature of 140 F.Bottom-hole pressure in the tubing string is 1,150 psi. Experience hasshown that it is necessary to heat the crude petroleum to a temperatureof 200 F. for it to reach the surface without having paraifin deposit onthe internal surfaces of the tubing string. To burn sufficient gas toincrease the temperature of the crude petroleum from F. to 200 F., it isnecessary to inject 800 to 1,000 s.c.f. of air per hour at a pressure of1,200 psi. The combustible mixture is burned in burner 34. Thecombustion products, plus additional unburned gas making a total ofabout 2,000 s.c.f. per hour, are introduced into petroleum tubing string20 at lifting-gas induction manifold 50 in zone 36. Approximately 6,000s.c.f. of hydrocarbon gas per hour passes upward through tube 38. At theinduction manifold in zone 40, approximately 2,000 s.c.f. of gas perhour is introduced into the petroleum tubingstring. The remaining gaspasses upward through the well-bore to be introduced into the tubingstring through additional gas-induction manifolds.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. In the recovery of petroleum through a well-bore penetrating asubterranean reservoir including an oilproducing stratum, la combustiblegas-producing stratum vertically spaced therefrom, and a substantiallyimpermeable layer lying between said strata, wherein viscous deposits ofpetroliferous material form in and plug a petroleum tubing-string insaid well-bore, the improvement comprising isolating zones of saidwell-bore adjacent to said oil-producing stratum and said gas-producingstratum from each other, collecting gas produced from said gasproducingstratum in said well-bore, pumping air down said well-bore at a pressureapproximating that of said gas to at least the depth at which saidpetroliferous deposits form in said tubing-string, burning said gas inthe presence of said air, and exhausting the combustion products upwardthrough said well-bore in heat-exchange relationship with petroleum insaid tubing-string.

2. The method in accordance with claim 1 including the step ofintroducing combustion products of said gas and air into said petroleumtubing-string.

3. The method in accordance with claim 2 in which said burning occurs ina zone of said well-bore adjacent to said impermeable stratum.

4. The method in accordance with claim 2 in which combustion productsare introduced into the petroleum tubing-string at a plurality ofvertically spaced points along said tubing-string.

5. The method in accordance with claim 4 in which unburned gas producedfrom said gas-producing stratum is also introduced into said petroleumtubing-string.

6. The method in accordance with claim 3 including the step ofisolating, by means of packers, a well-bore zone adjacent to saidoil-producing stratum, a well-bore zone adjacent to said impermeablelayer, and a well-bore zone adjacent to said gas-producing stratum, andintroducing a portion of the combustion products of gas and air intosaid petroleum tubing-string at a point within said well-bore zoneadjacent to said gas-impermeable layer.

References Cited in the file of this patent UNITED STATES PATENTS

1. IN THE RECOVERY OF PETROLEUM THROUGH A WELL-BORE PENETRATING ASUBTERRANEAN RESERVOIR INCLUDING AN OILPRODUCING STRATUM, A COMBUSTIBLEGAS-PRODUCING STRATUM VERTICALLY SPACED THEREFROM, AND A SUBSTANTIALLYIMPERMEABLE LAYER LYING BETWEEN SAID STRATA, WHEREIN VISCOUS DEPOSITS OFPETROLIFEROUS MATERIAL FORM IN AND PLUG A PETROLEUM TUBING-STRING INSAID WELL-BORE, THE IMPROVEMENT COMPRISING ISOLATING ZONES OF SAIDWELL-BORE ADJACENT TO SAID OIL-PRODUCING STRATUM AND SAID GAS-PRODUCINGSTRATUM FROM EACH OTHER, COLLECTING GAS PRODUCED FROM SAID GASPRODUCINGSTRATUM IN SAID WELL-BORE, PUMPING AIR DOWN SAID WELL-BORE AT A PRESSUREAPPROXIMATING THAT OF SAID GAS TO AT LEAST THE DEPTH AT WHICH SAIDPETROLIFEROUS DEPOSITS